This invention relates to fume hoods, and more particularly, to fume hoods which are equipped with auxiliary air chambers.
As explained more fully in U.S. Pat. No. 3,747,504, some fume hoods are equipped with auxiliary air chambers in order to decrease the amount of conditioned room air that is exhausted through the fume hood. Auxiliary air systems conventionally supply about 75% of the air which is exhausted by the fume hood, and the remaining 25% is drawn from the laboratory room to prevent fume counterflows from entering the room.
Most fume hoods include a sash which may be raised or lowered to close the working chamber of the hood. The top of the hood is provided with a clearance to permit the sash to be raised, and this opening presents a possibility of air leakage.
Further, we have found that fumes from within the hood may cling to the rear surface of the sash or may be carried or drawn upwardly by the sash as it is raised. As these fumes move upwardly through the sash clearance opening, they can escape into the laboratory.
The invention utilizes a pair of seals to prevent or inhibit air leakage through the sash clearance opening, and the rear seal also wipes the sash clean of any clinging fumes as it is raised, resulting in greater safety and efficiency. Auxiliary air which otherwise might escape through the opening is routed through the hood. When the sash is raised, the rear or upper seal contacts the rear surface of the glass. The seals may be provided in such sizes that when the sash is lowered the rear seal will contact the rear wall of the auxiliary air chamber and the front seal will contact the front wall of the working chamber. The rear seal will thereby prevent room air from entering the working chamber when the sash is raised, partially lowered, and completely lowered and will wipe the sash of clinging fumes as the sash is raised. The front seal will prevent loss of auxiliary air through the sash clearance opening when the sash is raised, partially lowered, and completely lowered.